Re: [PATCH] irqchip/gicv3-its: Don't allow devices whose ID is outside range

[Shanker Donthineni]

Hi Marc,

On 02/23/2016 02:51 AM, Marc Zyngier wrote:
> On 23/02/16 03:57, Shanker Donthineni wrote:
> > We are not checking whether the requested device identifier fits into
> > table or not. The ITS MAPD command fails if 'Device ID' is outside of
> > device table range.
> >
> > Add a simple validation check to avoid MAPD failures since we are
> > not handling ITS command errors. This change also helps to return an
> > error -ENOMEM instead of success to caller.
> In which circumstances do you see this failing? We allocate memory for
> the whole DevID range (as advertised by the ITS), so anything that comes
> up outside of that range cannot be handled anyway, and is a HW
> integration issue. You might as well pretend that these devices do not
> exist.
>
> Thanks,
>
> 	M.
We will try to allocate maximum memory as much as possible to cover 
whole DevID.
sparse. We may not always successful, sometimes we limit memory 
allocation size
for two reasons MAX_ORDER and ITS flat table capacity.

1) According to ARM-GIC spec, ITS hw can access maximum of 256 (ITS-pages) *
   64K (ITS-pageszie) bytes.

    21bits = 16 MBytes / 8 (flat table size / device table entry size)

Assuming: minimum device table entry size 8 Bytes

2) On 4K page size kernel with default arch/arm64/defconfig.

   19bits = 4 MBytes / 8 (flat table size / device table entry size)

Assuming: minimum device table entry size 8 Bytes

This is definitely a problem if DevID range is much more than 19Bits and 
we fail to allocate
enough memory. Our ITS hardware has capable of supporting all 32bits, so 
it advertises
DevID 32.

I am preparing an another patch to support ITS-Indirection (two-level 
table walk) feature.

--
Shanker Donthineni
Qualcomm Technologies, Inc. on behalf of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project